import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution2 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @param m int整型 
     * @param n int整型 
     * @return ListNode类
     */
    public ListNode reverseBetween (ListNode head, int m, int n) {
        // write code here
        ListNode dum = new ListNode(0);
        dum.next = head;
        ListNode cur = dum;
        while(m > 1) {
            m--;
            cur = cur.next;
            n--;
        }
        ListNode pre = cur;
        cur = cur.next;
        while(n > 1) {
            n--;
            ListNode curNext = cur.next;
            cur.next = curNext.next;
            curNext.next = pre.next;
            pre.next = curNext;
        }
        return dum.next;
    }
}